When a passenger rides on the rear seat of a two-wheeled vehicle, the passenger puts both arms around the body of a driver, or holds a device, such as a holding belt provided between the driver's seat and the rear seat or a holding grip provided at the side or the back of the rear seat to ride on the vehicle, thereby maintaining the stability of the riding posture and preventing the passenger from falling from the two-wheeled vehicle at the time of starting and running a curved road. Moreover, like the belt with a grip for tandem riding on a two-wheeled vehicle disclosed in following patent document 1, a driver attaches a device in a belt shape around the body of the driver, and a passenger holds the holding grips provided at both ends of the belt, thereby maintaining the stability of the riding posture and preventing the passenger from falling from the two-wheeled vehicle.
However, such devices like the holding belt and the holding grip attempt to maintain the stability of the riding posture based on the gripping power of the passenger. Hence, it is difficult for women and children with a weak grip power to support the upper part of the body for a long time using the holding belt and the holding grip. Moreover, a function of suppressing a falling of the passenger from the two-wheeled vehicle is not ensured when the passenger falls asleep or releases the hand from the holding belt and the holding grip.
Following patent document 2 discloses a falling prevention device which maintains the stability of the riding posture of the passenger not by only the gripping power of the passenger riding on the rear seat of a two-wheeled vehicle. This device employs a configuration in which the driver and the passenger are coupled together through a double-structure waist belt, thereby maintaining the stability of the riding posture of the passenger. However, the stability of the riding posture of the upper part of the passenger from the waist needs to be supported by holding a holding belt or a holding grip, etc. Hence, when, in practice, the passenger fails to hold the holding belt, the holding grip, etc., by gripping power due to, for example, catnap, no structure is employed which maintains the stability of the riding posture of the upper part from the waist, and it is unavoidable that the passenger largely bends the body backwardly or tilts the body from side to side.
A safety belt device for tandem riding on a two-wheeled vehicle typical of a scheme disclosed in patent document 3 prevents the passenger from falling from the two-wheeled vehicle while maintaining the stability of the riding posture of not only the waist of the passenger riding on the rear seat of the two-wheeled vehicle but also the whole upper part of the body. This device includes a waist belt fastening respective waists of the driver and the passenger so as to tie both waists together, and a pair of shoulder belts fastening respective shoulders of the driver and the passenger tied together. According to such a device, the stability of the riding posture of the upper part of the passenger from the waist can be maintained without depending on only the gripping power of the passenger, thereby preventing the passenger from falling from the two-wheeled vehicle.
According to such a device, however, when the difference in physical size between the driver and the passenger is large and the seating position for the driver and the seating position for the passenger have the same height, as shown in FIG. 21, the device cannot be surely attached to the shoulder of the passenger, and the upper body of the passenger may be released from the device due to a rolling when the two-wheeled vehicle is running Moreover, according to such a device, when the difference in physical size between the driver and the passenger is substantially zero or when the physical size of the passenger is larger than that of the driver and when the seating position for the passenger is higher than the seating position for the driver, as shown in FIG. 22, the shoulder belt interferes with the neck of the driver, which may disturb the driving.